Lithography systems are commonly used to transfer images from a reticle onto a semiconductor wafer during semiconductor processing. A typical lithography system includes an optical assembly, a reticle stage for holding a reticle defining a pattern, a wafer stage assembly that positions a semiconductor wafer, and a measurement system that precisely monitors the position of the reticle and the wafer. During operation, an image defined by the reticle is projected by the optical assembly onto the wafer. The projected image is typically the size of one or more die on the wafer. After an exposure, the wafer stage assembly moves the wafer and then another exposure takes place. This process is repeated until all the die on the wafer are exposed. The wafer is then removed and a new wafer is exchanged in its place.
Immersion lithography systems utilize a layer of immersion fluid that completely fills a gap between the optical assembly and the wafer during the exposure of the wafer. The optic properties of the immersion fluid, along with the optical assembly, allow the projection of smaller feature sizes than is currently possible using standard optical lithography. For example, immersion lithography is currently being considered for next generation semiconductor technologies including 65 nanometers, 45 nanometers, and beyond. Immersion lithography therefore represents a significant technological breakthrough that will likely enable the continued use of optical lithography for the foreseeable future.
After a wafer is exposed, it is removed and exchanged with a new wafer. As currently contemplated in immersion systems, the immersion fluid would be removed from the gap and then replenished after the wafer is exchanged. More specifically, when a wafer is to be exchanged, the fluid supply to the gap is turned off, the fluid is removed from the gap (i.e., by vacuum), the old wafer is removed, a new wafer is aligned and placed under the optical assembly, and then the gap is re-filled with fresh immersion fluid. Once all of the above steps are complete, exposure of the new wafer can begin.
Wafer exchange with immersion lithography as described above is problematic for a number of reasons. The repeated filling and draining of the gap may cause variations in the immersion fluid and may cause bubbles to form within the immersion fluid. Bubbles and the unsteady fluid may interfere with the projection of the image on the reticle onto the wafer, thereby reducing yields. The overall process also involves many steps and is time consuming, which reduces the overall throughput of the machine.
An apparatus and method for maintaining immersion fluid in the gap adjacent to the projection lens when the wafer stage moves away from the projection lens, for example during wafer exchange, is therefore needed.